字符串为空判断

    NSString *trimedString = [textField.text stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];
    if ([trimedString length] == 0) {
        UIAlertView *alert = [[UIAlertView alloc]initWithTitle:@"警告" message:@"搜索内容不能为空" delegate:self cancelButtonTitle:@"确定" otherButtonTitles:nil, nil];
        [alert show];
        [alert release];
        return true;
    }